Thermionic converter having a crystallographic orientated emitter



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H. F. WEBSTER 3,303,362 THERMIONIC CONVERTER HAVING A CRYSTALLOGRAPHIC v ORIENTATED EMITTER Filed Nov. 25. 1964 2 Sheets-Sheet 1 Power W G 0 Q I /nvenf0r;

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3,303,362 APHIC 2 Sheets-Sheet 2 70 Cesium Reservoir Temperature ERTER HAVING A CRYSTALLOGR ORIENTATED EMITTER Degrees Cent/grade for each Curve refer f e I 5 m. W W l 2 3mm 0 H a WT by Q H/s Attorney.

Feb. 7, 1967 3 H. F. WEBSTER THERMIONIC CONV Filed Nov. 25, 1964 v T V 6543 2 654 3 2 0 Fig. 5.

United States Patent 3,303,362 THERMIONIC CONVERTER HAVING A CRYSTAL- LOGRAPHIC ORIENTATED EMITTER Harold F. Webster, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Nov. 25, 1964, Ser. No. 415,566

5 Claims. (Cl. 31tl4) This application is a continuation-in-part of my application Serial No. 140,815, now abandoned, filed September 26, 1961, entitled, Thermionic Device and Method of Manufacture, and assigned to the assignee of the present invention. The invention relates to an electric discharge device and, more particularly to a vapor filled thermionic converter device including an advantageous cathode construction.

A thermionic discharge device of the type contemplated herein comprises a pair of members forming emitter and collector electrodes, respectively, separated by a small distance. The emitter or cathode is heated to a temperature sufficient for copious electron emission and the collector or anode serves to collect such emitted electrons. An external load circuit may be connected across these electrodes to provide a completed conductive loop for electron flow, and utilization of converted power.

As may be understood, factors affecting the performance and efficiency of thermionic power converters and the like are the emission rate of the cathode and the impedance to movement of the electrons in the interelectrode space from cathode to collector, after emission. In evacuated converters, the space charge, produced by electrons emitted from the cathode, becomes an appreciable impediment to the movement of electrons newly emitted from the cathode.

A known expedient for minimizing the effects of space charge is to fill the inter-electrode region with cesium gas. During operation of the device, the cesium gas becomes ionized filling this region with positively charged ions to minimize or eliminate the space charge effects. Most of these ions are formed at the heated emitter electrode by surface ionization and tend to establish an ion equilibrium condition with the cesium atmosphere.

A percentage of the emitter surface remains covered with an average number of cesium atoms as governed by cesium pressure and emitter temperature. These atoms have the desirable effect of lowering the effective work function of the emitter; that is these atoms lower the energy a current-carrying electron must attain before it leaves the emitter for the collector to provide greater emission. Greater electron emission may then be obtained by increasing the cesium gas pressure within the inter-electrode region of the device to increase the number of atoms. However, as a practical matter this expedient has its limitations in achieving high current or high electron transfer between emitter and collector. This is so because increased gas pressures usually cause significant scattering of electrons in their travel from the cathode toward-the collector due to collisions with gas ions and atoms. Scattering reduces the percentage of emitted electrons that are collected at the collector and therefore lowers the efficiency and output capacity of the device. Plasma voltage drop takes place inthe converter device and tends to be higher at higher gas pressures, reducing the power deliverable to a load.

An additional method of raising the output is to raise the emitter temperature, However, the temperature of the emitter cannot ordinarily be raised in an unlimited manner because of practical considerations of handling the heat and because raising the emitter temperature tends to keep the work-function-reducing material boiled off" the cathode.

3,393,362 Patented Feb. 7, 1967 In accordance with the present invention, other phenomena are uniquely exploited to greatly increase electron emission of a thermionic device. The work function of the cathode surface depends not only on the number of cesium atoms impinging on it but also on the crystallographic surface to which the cesium adheres. It may be postulated that greater binding forces between the cathode surface and the cesium ions result when the proper crystal plane coincides with the surface of the planar emitter. The greater such binding force, the easier cesium adheres to the emitter surface resulting in a decreased effective work function. I

A feature of my invention involves the structure of faces of crystal material forming acathode which provide unusually high emission in a thermionic converter. Since this construction does not require the use of relatively high gas pressures, the adverse eifect of high electron scattering with attendant voltage drop is avoided. However, the postulated better adherence of the cesium as a coating enables operation of the cathode at quite high temperatures, further enhancing electron emission.

Likewise, a thermionic converter in accordance with the present invention operating at temperatures and pressures similar to those heretofore employed is capable of on the order of a tenfold increased output current density and an efficiency increase from 15% to approximately 25%.

It is an object of my invention to provide an improved thermionic device delivering enhanced current densities and, therefore, larger power outputs at improved efficiencies.

It is another object of my invention to provide an improved thermionic device which delivers greater and more uniform emission at the emitter thereof.

It is another object of my invention to provide an improved thermionic device operating at greater current densities for given gas pressures and operating temperatures.

The novel features believed characteristic of the pres ent invention are set forth in the appended claims. The

invention itself, together with further objects and advantages thereof, may be understood with reference to the accompanying drawing wherein:

FIG. 1 is a vertical cross section of an embodiment of a thermionic converter device in accordance with the reciprocal converter emitter temperature in accordance with the present invention, and

FIG. 6 is a partially broken away view of apparatus for conducting a process of forming electrodes in accordance with the present invention.

Referring to FIG. 1, illustrating a thermionic converter device in accordance with the present invention, a heated emitter electrode 1 is disposed opposite a cooler collector electrode 2 in close spaced relation, on the order of to ,6 of an inc-h therefrom. Emitter 1 is formed of a refractory, low vapor pressure metal and is joined both mechanically and electrically to an end plate 3 formed of similar metal. Collector 2 also formed of a refractory, low vapor pressure metal is joined mechanically and electrically to the remaining end plate 4, formed of the same or similar material. The complete device is conveniently cylindrical, employing an annular ceramic spacing member 5 for providing the outside wall for the converter.

The bond between the ceramic and the electrodes may be made in accordance with any of the known processes, due consideration being given to the temperatures to be encountered in the operation of the device and the required resistance of the resulting structure to attack by the metal vapor to be employed. For example, the ceramic may be first metallized by a process known as the manganese-molybdenum process and then further plated and bonded to the electrode members. Such a process is described in detail and claimed in Patent No. 2,667,427 to Nolte, dated January 26, 1954. Alternative bonding methods as well as further details of general converter construction and operation may be found in the copending application of Volney C. Wilson for Method and Apparatus for the Direct Conversion of Thermal to Electrical Energy, Serial No. 698,552, filed November 25, 1957, and assigned to the assignee of the present invention.

A reservoir for holding a quantity of readily vaporizable metal material is provided by a tubulation 9 in which a small quantity of such material 10 is retained. Such material is an alkali metal which is at least partially vaporizable when maintained at a somewhat elevated temperature. The met-a1 may be, for example, cesium or rubidium, but cesium is preferred. The tu'bulation 9 communicates at one end with the space between the emitter and collector electrodes to supply the alkali metal vapor thereto at a pressure dependent upon the temperature of the coolest surface to which the tu bulation is exposed.

The following are examples ofrefractory metals from which the emitter and collector electrodesmay be formed. This list is not necessarily all-inclusive. The body centered cubic materials tungsten,.tantalum and molybdenum are suitable. Face centered cubic materials such as nickel and iridium are also appropriate as are hexagonal lattice materials such as rhenium and zirconium. Alloys of the various materials are also useful such as, for example, alloys of tungsten and rhenium. The emitter electrode metal should have a high work function in the absence of the alkali metal vapor employed, as compared wit-h the ionization potential of such metal vapor. The uncoated work functions of preferred 'metals tungsten, tantalum, molybdenum, rhenium and iridium are about 4.5, 4.2, 4.2, 5 and 5.3 volts respectively and are high with respect to or significantly greater than the 3.89 volt ionization potential of cesuim. -In general the difference is prefera bly at least three-tenths of a volt. In accordance with an important feature of the present invention, it is desirable to employ a single crystal of such met-a1 as the emitter electrode surface or an element thereof and to dispose such single crystal such that a particular face is oriented towards the collector electrode as hereinafter more fully set forth. V

End plate 4 has bonded thereto at its periphery a hollow cylindrical jacket 6 having inlet and outlet passages 7 and 8 for directing coolant inheat exchange relation with the end plate 4 of collector electrode 2. Cooling liquid, or gas, for example, water, liquid metals or air flowing in hollow cylindrical jacket 6 maintains the collector elect-rode 2 at a lower temperature than the heated emitter electrode. A similar cooling arrangement (not shown) may also be used to maintain alkali metal 10 at a still cooler temperature being above 200 C. but less than the temperature of said collector electrode.

In operation athermionic converter embodiment such as illustrated in FIG. 1 has its emitter electrode heated by a source of thermal energy illustrated schematically by the arrows in FIG. 1. Such heat may be concentrated solar heat or may be obtained from a hot member of a nuclear reactor. As far as the device itself is concerned, a simple burner may provide the heat energy required. In any case the emitter electrode should be heated to a temperature above 1000" K. and preferably above 1600 K. to provide surface ionization for the alkali metal vapor. The temperature should be less than 2500 K. Since the collector electrode 2 is maintained at tempera this material.

4 ture cooler than the emiter electrode, electron emission will take place from the emitter to the collector and may be caused to flow in an external circuit illustrated by resistor 11 in FIG. 1 connected between end plates 3 and 4. In such case the end plate connected to the hot-ter of the electrodes, i.e., end plate 3, will be positive with respect to plate 4.

The alkali metal vapor in the area between the electrodes 1 and 2 becomes ionized principally at the heated emitter electrode 1 and an ion equilibrium condition tends to establish between the surface of emitter electrode 1 and the alkali metal vapor atmosphere. The ions thus created neutralize the electron space charge which would otherwise exist between the electrodes in the absence of such alkali metal vapor and which would impede current flow. Furthermore, the alkali metal vapor coats opposing surfaces of both the emitter and collector with what is thought to be at least a partial monatomic layer of atoms and ions on the surface of each electrode. The collector electrode being at the lower temperature is coated with the alkali metal more completely than is the emitter electrode at the higher temperature because the heat of the emitter electrode tends to drive off more of The coated work function of the emitter is then greater than the work function of the collector whereby a voltage difference exists therebetween. It is postulated that the layer of alkali material, for example, cesium, establishes a dipole layer upon the electrode surfaces, lowering the surface barrier which electrons must exceed before being emitted from such electron. Thus positive cesium ions disposed on the emitter surface facilitate the out-flow of electrons from the emitter 1 to the collector 2. The work function of the emitter is thus lowered.

The difference in the work functions between the emitter and collector electrodes measured in electron volts is called the contact potential between the two electrodes,

and provides the output voltage of the device neglecting max s V wherein:

I is the current flow equal to the saturated emission density times the area of an electrode is the work function of the emitter 5 is the work function of the collector Thus, it appears that although a difference in the work function must exist, one should also adjust the current density to maximize power output. Often the latter consideration is the larger consideration in magnitude so, if necessary, one may sometimes sacrifice contact potential in favor of an increase in current density.

An increase in current density is achieved in accordance with the present invention, obtaining a current density on the order of 10 times that heretofore known in thermionic converter devices, thereby delivering a considerable increase in power output for a thermionic converter. This increase is achieved by providing a proper crystallographically oriented face on the emitter electrode over a substantial portion of the 'emitter electrode. surface oriented towards the collector electrode. Preferably the emitter electrode is a single crystal of refractory metal, the metal being, for example, one of those previously mentioned. The face found'to deliver the largest increase in emission density and therefore power output in accordance with the present invention is that face which exhibits a very high or the highest work function in the absence of the alkali metal vapor, in comparison to other crystallographic faces. Then, in the presence of the alkali metal vapor emitting surface, the said face is found to exhibit a lower work function than other crystallographic faces. Moreover this face is found to provide an increased density of electron emission and an increased density of emission transfer to the anode over the surface of the emitter electrode.

It is postulated this increased current density is due to better adhesion between the alkali metal emitting coating and the particular face, that in the absence of the alkali metal would exhibit a higher work function. The surface which in the absence of the alkali metal exhibits a high work function is characterized by close atomic spacing between the atoms of the metal crystal on the face of the crystal. It is postulated that in the absence of alkali metal vapor or the like, the atomically close packed surface permits the escape of fewer electrons from the uncoated surface. However, in the presence of atoms and/or ions of alkali metal material, a dipole layer is set up which decreases the surface barrier. When a large number of charges per unit area exist on the atomically packed surface, the large number of charges per unit area is capable of holding a more uniform layer of alkali metal submolecular particles for a longer period of time at high temperatures. Thus increased adhesion of alkali metal coating for the atomically closed packed surface decreases the work function barrier which electrons must overcome before leaving the surface to a larger extent than heretofore known in thermionic converters, and moreover greatly increases the current density of electrons transmitted from this surface to the collector electrode.

Alternatively, other theories may be advanced to explain the improved emission from such atomically close packed surface. For example, the contribution per alkali metal atom to increased emission may be higher. That is, the alkali metal atoms may be held to the emitter surface at a greater distance therefrom to provide a larger dipole moment and greater acceleration for electrons escaping from the emitter surface toward the collector. In any event, it should be understood applicant is not bound by a particular theoretical explanation for the improved performance of the present device.

The preferred surface which has been described as being that surface which in the absence of alkali metal coatings has a high work function and which is atomically close packed, may be described differently for different refractory metals. For a single crystal of metal having a body centered cubic lattice, the preferred direction of exposure may be described as the 110 crystallographic face. The number 110 refers to the well known Miller indices of such face. In accordance with the science of crystallography, the body centered cubic lattice, e.g. of tungsten, has a repeated crystal structure of the cube with an atom at each of the eight corners of the cube as well as one in the middle of the cube and touching the other eight. This structure is repeated with the side of one cube forming the side of the next cube in the lattice, etc. However, the sides of the cube do not necessarily present a most favorable surface for emission in accordance with the present invention. The surface found to be most favorable for high emission density in a thermionic converter, the 110 face, is that arrangement of atoms which is revealed if the cube is substantially bisected in a planar direction extending from one edge of the cube to the diagonally opposite edge, leaving the center atom in position, this planar direction being also parallel to the two remaining edges. In other words, the face described is the face left when two atoms forming a single edge of the cube are removed.

Referring to FIG. 4A, illustrating the 110 face of a large crystal lattice, such a face is illustrated in its relative packing density in comparison to the packing along the sides of complete cubes, FIG. 4B, or the crystallographic arrangement revealed if the 111 surface of the lattice is exposed as illustrated in FIG. 4C. If the distance from one atom to another across the edges of a complete cube is given as unity (FIG. 4B), then the numbers given in FIG. 4A and FIG. 4C illustrate the relative distances between centers of atoms as compared with one.

The present invention is not restricted to the most closely packed face but it will be apparent that a somewhat less closely packed face will frequently provide material improvement over prior art converter arrangements. For the body center cubic lattice the 112 face is sometimes found to be the next most satisfactory, being the next most closely packed surface.

For face centered cubic lattice crystals, the face having the ill crystallographic direction is the most closely packed and therefore the most satisfactory providing highest current density in a thermionic converter according to the present invention. This is also the surface having the highest work function in the absence of alkali metal vapor. The face is frequently the next most satisfactory.

For hexagonal lattice crystals the most desirable face, is the 0001 face; the next most satisfactory faces are the 1010 and 1011 faces.

It is desirable to employ an emitter electrode surface comprising a single unitary crystal of the material, since single crystals are presently readily available, for example, for a material such as tungsten. However, it is understood the present invention is not limited to emitters com prising a single crystal. Often satisfactorily improved operation will be obtained for materials having a large or major proportion of crystal facets of the desired packing in the desired direction. Therefore the devices as disclosed herein may have preferred orientation electrode material substituted wherever a single crystal elect-rode is called for. Materials of this type can be formed in a manner hereinafter more fully set out.

Referring to FIGS. 2 and 3 illustrating another embodiment of the present invention wherein FIG. 3 represents a cross-section taken at AA in FIG. 2, a single crystal of refractory metal having a square cross-section forms an emitter 12 surrounded and spaced from a collector 13, the said emitter being disposed in an aperture of rectangular cross-section in collector 13, this aperture accommodating the emitter with a spacing of approximately 100th of an inch or less on all sides thereof. The emitter material employed may conveniently be a single crystal of tungsten having its 001 crystallographic direction axial to the emitter. The crystal is cut to have a crystallographic face on each lateral side thereof opposite and facing the inside of the collector aperture walls. The collector is larger in mass than the emitter for convenience of heat dissipation and may be conveniently formed of tungsten, molybdenum, nickel, alloys of zirconium, or other metals. A hollow coolant jacket 14 surrounds collector 13 and is conveniently provided with circulating water or some other liquid or gas for collector cooling purposes by means not shown. A reservoir for holding a quantity of readily vaporizable material is provided by tubulation 15 communicating with the space between emitter 12 and collector 13 and having a depending end at 16 for holding the readily vaporizable material 17. Such material is an alkali metal, partially vaporizable when maintained at a somewhat elevated temperature. The metal may be cesium or rubidium. Depending portion 16 is surrounded by a second hollow jacket 18 into which coolant may be conveniently circulated for maintaining the metal 17 at a temperature below that of collector 13.

The emitter 12 is supported within the collector with an annular ceramic spacing member 18 at the lower end of the emitter and collector. The collector is counter sunk at this point, leaving a greater spacing between emitter and collector to accommodate ceramic member 18 having a greater radial dimension than the normal spacin-g between the emitter and collector. The ceramic member 18 is formed of the same material and bonded to the electrodes in the same manner as hereinbefore set out in connection with the embodiment of FIG. 1. This spacing arrangement is illustrative only and other spacing arrangements, for example for compensating expansion in high temperature devices, will occur to those skilled in the art. A load represented by resistor 19 is electrically connected between the emitter 12 and the collector 13.

The center of emitter 12 is hollowed to receive a cylindrical tantalum sleeve 20 which may be closed at each end if desired and which may contain a heat source 21. The heat source 21 may, for example, comprise a material undergoing nuclear fission. However, it is apparent that any suitable heat source can be substituted therefor which will heat the electron emitted 12 to the temperature beween 1000" and 2500 K. The collector .13 is maintained at a lower t-meperature by means of a coolant in jacket 14, while the reservoir of alkali metal is maintained at a temperature between approximately 200 C. and the temperature of said collector. Appropriate temperatures are hereinafter more fully described 7 The embodiment of FIGS. 2 and '3 has the advantage of utilizing more fully the preferred crystal surfaces of a single crystal emitter and moreover of providing a collector which is more easily cooled. Furthermore the efficiency of heating the cathode is greater because heat escapes primarily only in a direction toward the converter gap. This arrangement is also convenient for connecting a number of such thermionic converters in serial arrangement. In such an arrangement the collector electrode 13 of one converter is joined to the emitter of a second such converter placed therea bove. In such an arrangement it is convenient to employ one common alkali metal vapor circuit bewteen a plurality of converters while receiving alkali metal vapor from a single reservoir wherein such reservoir is maintained at the lowest temperature in the vapor circuit to regulate the alkali metal vapor pressure to a desired value.

The body centered single crystal emitter electrode 12 is cut to have the 001 crystallographic direction in a direction longitudinal to the electrode and having the four side faces oriented in the 110 crystallographic direction. In thismanner, cesium material tends to adhere well to the emitter faces at elevated temperatures above 1000 K. and preferably above 1600 K., enabling a considerable increase in emitter power over thermionic converters known heretofore.

FIG. illustrates a set of curves plotting current density in amps per cm. of emission from a single crystal tungsten emitter versus the quantity 1000/T where T is in degrees Kelvin. Therefore, temperature is seen to increase from right to left along the abscissa. The different curves are for various cesium reservoir temperatures wherein the temperature of the cesium in the tubulation provided in the hereinbefore described converters, is determinative of the cesium pressure between the emitter and collector electrodes. Thus, a temperature of 200 C. the temperature of the lower curve (i.e.,

' 473 K.) is approximately equivalent to a cesium vapor pressure of .09 mm. Hg. It is appreciated that operation of such a converter may take place at convenient cesium pressures in the range between approximately 200 C. cesium reservoir temperature and 350 C. cesium reservoir temperature although the lower temperatures and pressures in this range are preferred; other curves may be interpolated for such other temperatures between those shown in FIG. 5. v

The abscissa scale between .4 and 1.0 corresponds to the emitter temperature range from 2500 K. on the left to 1000 K. on the right. These curves are similar to the well known Taylor-Langmuir S-curves which may be drawn for prior art vapor converters. However, it should be noted that for a given cesium reservoir temperature (and therefore for a given cesium pressure) and for a given emitter temperature, an approximately '8 tenfold increase in current density above that given by Taylor-Langmuir curves is attained. Likewise an increase in current density is possible at reduced cesium pressures, the reduced pressures resulting in reduced electron scattering in the converter. Reduced scattering increases the output voltage, or alternatively allows greater spacing between emitter and collector electrodes for easier construction of the device. At lower cesium pressures, arc-drop voltage loss may be reduced or virtually eliminated in the converter device according to the present invention. The cesium reservoir temperature in this low scattering mode of operation is desirably below 277 C. or below a cesium pressure of 1.0 mm. of mercury. The emitter temperature for low scattering operation should be such that sufficient surface ionization takes place thereat. This temperature is preferably above It might at first be thought that a generally preferred point of operation would be nearer the lower temperature range near the right end of the various curves in FIG. 5. However, it is found that lower temperatures near the curve peaks, although apparently resulting in increased emission density, do not result in corresponding increased transmission to the collector electrode. The reason is insufficient ionization to satisfactorily eliminate the space charge near the peak of these curves and therefore an increase in voltage drop across the converter. Near the lower temperatures, the coated work function of the emitter is found to decrease too far below the ionization potential of the cesium vapor and fewer ions are provided to neutralize the space charge. Also as the work function differential between emitter and collector becomes smaller, the output voltage .de-

creases. I

On the other hand, although the emitter work function increases at increased temperatures towards the left end of the curves, it is seen that current density is decreasing. Furthermore, unusually high emitter tempera tures are encountered at the left end of the curves.

It is also not generally advisable to raise the cesium reservoir temperature beyond the values given on the top curve even though current density appears to increase with cesium reservoir temperature, since greatly increased cesium pressure tends to increase the scattering and reduce the actual emission that reaches the collector while also reducing the output voltage.

A compromise between high and low emitter temperatures is desirable since lower emitter temperatures result in a lower work function and higher temperatures result in a lower emission density. The dashed straight line on the graph labelled J /J and having a negative slope is indicative of the general region wherein the converters according to the present invention have been found to be most effective. This line corresponds very roughly to a coated emitter work function of approximately 2.8 electron volts. As will be appreciated by those skilled in the art, this is below the ionization potential which may be given for cesium. However, sulficient ionization takes place in this region and even somewhat to the right thereof to provide satisfactory space charge neutralization. The line is drawn for an ion concentration of .002 ions per electron, i.e. 500 electrons per ion. Considerably less than this number of ions will satisfactorily neutralize the space charge insofaras satisfactory operation of'the present device is concerned. In the equation for which the dashed line of FIG. 5 is drawn, 1 relates to the number of ions on the emitter and equals:

where 1 is ions measured in amps per square centimeter, e is charge of an electron equals 160 10 coulombs, I is the ionization potential of the gas (cesium equals 3.89 electron volts), is the work function of the coated surface, K is Boltzmans constant, e/K equals 11,606, T is the temperature in degrees Kelvin, and P is the pressure of cesium vapor in mm. Hg. This is the Langmuir- Saha equation.

In the equation for the dashed line in FIG. 5, L is the electron concentration in amps per square centimeter given by the Richardson-Bushman equation Where the factors have the same meanings as before.

The power optimization for a particular converter device may depend upon the actual converter dimensions and specifically the emitter area over which emission occurs. One applies Expression 1 to determine power optimization for the particular device. Ordinarily, optimization Will occur near the aforementioned ion concentration line.

This is not to say that converter operation is not possible at other points along the curves given (or of course along curves interpolated between those given). For example, although ion concentration due to surface ionization at the emitter sometimes decreases quite rapidly to the right of this line, it has been found that even volume ionization effects in an area between the electrodes can enable operation in some cases near the peak current density for the curves occurring at the lower temperatures. This is especially true if auxiliary means are employed to produce ions in the interelectrode region. One such auxiliary ararngement for producing ions in the area between the electrodes is set forth and claimed in the aforementioned copending patent application of Volney C. Wilson.

In addition to the other advantages herein set forth, emission from a primarily single-crystalline cathode surface is relatively uniform in velocity as compared with prior art cathodes, whether the electrode is coated with emitting material or not. This results in decreased noise in an electric discharge device including an anode and a single-crystalline cathode.

In order to optimize the output in a converter device, it is also desirable to provide collector surface with a preferred exposed face. Since the power output is proportional to the difference in work functions between the emitter and collector, it is desirable to lower the collector work function so that this difference will be the greatest. As might first be thought, such reduction in work function might occurif the same crystallographic face is chosen for the collector as produces the lowest Work function for the emitter in the alkali metal vapor atmosphere. However, it has been frequently found that the collector work function is lower if a less closely packed face is chosen for exposure in the direction of the emitter. Thus, for body centered cubic metals such as tungsten or tantalum, a 100 face is desirable for the collector surface. For face centered cubic metals, a 100 face produces best results. Likewise, for hexagonal lattice materials, the 1010 face and the 1011 face produced the most desirable results. Of course, the difference in the preferred faces between the emitter and the collector is related to the difference in temperature at which the two are operated. Generally, the collector temperature will be in the neighborhood of 1000 K. or less. In this temperature range it has been found that a secondarily closely packed surface exhibits the lowest Work function in the alkali metal vapor atmosphere. It is tentatively theorized that the bond of the alkali metal vapor atoms and ions to the surface occurs at 'a distance related to the inter atomic spacing of the base refractory metal. As hereinbefore stated, the coating of alkali metal forms a dipole layer on the refractory metal surface. At higher temperatures such as those at which the emitter is operated, this bond must be relatively close to the surface of the refractory metal as compared with the bond which might form on other surfaces, in order that the alkali metal coating be held or bonded closely to the refractory metal at the high temperatures. However, at lower temperatures such as those at which the collector is operated, a slightly less closely packed surface may be capable of bonding the alkali metal atoms and ions, providing the dipole layer with a dipole moment greater than would be found on the more closely packed surface at such lower temperature, and therefore providing a lower work function at such lower temperature. Of course it isnt essential the collector be single crystalline.

The desired crystallographically oriented metal surface for the emitter or other electrode may be produced with polycrystalline materials by exposing a large number of grains with the desired crystal face. Preferred crystal growth can be produced by rolling, drawing and heat treatment (e.g. as disclosed and claimed in copending Dunn and Webster application, Serial No. 334,049), or by vapor deposition and the like. Some of these methods have been known to produce grain orientation in the direction for body centered cubic material, for example. The exposed face should be fiat as possible and free of non-uniform contaminants.

Of course, the most desirable surface is obtained with 'a single crystal or a flat surface consisting predominantly of grains with a selected face oriented toward the collector. Starting out with a single crystal of tungsten, illustrated as a completed emitter 12 in FIGS. 2 and 3, the location of a proposed 110 face can be determined by X-ray diffraction in a well known manner employing the back scatter Laue reflection pattern. Then the single crystal is machined and ground in the proper direction to expose an entire surface or surfaces in the 110 direction.

If a single crystal is grown to have its axis in the 001 direction, for example, then it can be cut to have four 110 type lateral faces. This is the manner in which the emitter electrode 12 of FIGS. 2 and 3 may be initially machined.

Unfortunately, grinding and machining, while revealing the approximate desired face, freqently expose a face at some small angle to the desired face. Inter-atomic spacings in the crystal lattice are so small compared to the accuracy of the machining operation that exact exposure of the array of atoms desired is somewhat difficult. Furthermore, the machining operation tends to damage the exposed face and disarrange the surface atoms somewhat. It is therefore desirable to provide a process for exposing in a more exact manner the face or facets desired. An advantageous process comprises etching an electrode surface under low voltage conditions to remove material to the desired face or facets. Although it is possible to etch a body of polycrystalline material or a body comprising a plurality of grains in this manner, it is desirable to start with a single crystal of electrode material which has been machined as closely as possible to expose the desired crystallographic direction.

Referring to FIG. 6 illustrating the process for etching the electrode surface, an electrode blank 22 is first machined to reveal the desired surface as closely as the machining will permit and then this blank is placed with the machined face upward on a pedestal 23 in a tank 24 containing an alkaline solution 25. An electrode 26 is disposed in the same alkaline solution and preferably above the desired face of blank 22. A low voltage power source 27 is connected between the electrode 26 and the blank 22 with the positive terminal connecting to the blank 22. This power source 27 is preferably a low voltage power source capable of supplying a fairly high current, depending upon the area of the electrode. A voltage between 1 and volts is satisfactory.

In a specific example, an electrode blank 22 shown horizontal in the drawing, is machined as closely as possible to reveal at least an upward face in the 110 direction. Power source 27 is adjusted to a preferred voltage between 1 and 3 /2 volts with its positive terminal con necting to electrode blank 22 and the negative terminal connected to electrode 26, the latter being formed of stainless steel. Such power source is energized for a period of several minutes and acts to cause etching of the electrode blank 22. One to three minutes at two or three volts is usually satisfactory. In a specific example the alkaline solution was a solution of sodium hydroxide having a concentration of 100 grams per liter of water, this concentration not being too critical.

The low voltage causes the current to pass between the electrodes causing etching of the electrode blank 22. As an end result in the example, stepped facets of the 110 surface were exposed on the electrode surface. These facets covered nearly the whole surface of the tungsten electrode exposed to the solution, with only very small steps or risers in between the successive 110 surfaces having a crystallographic direction other than 110. For best results it is desirable that the power source voltage be less than 3 volts, since above this voltage a certain amount of polishing of the material results producing an average surface of the same general orientation as the machining. It is also preferred to use a voltage greater than one volt since lower voltages consume considerably more time in producing the desired surface. Of course, when etching has provided the desired surfaces on the electrode 22, the electrode may be turned over to etch the remaining side. It is not strictly necessary that the surface to be etched be opposite the electrode 26, although such placement is desirable. The electrode 22 may alternatively be suspended in the solution from its attaching conductor so that all sides thereof are etched simultaneously.

The foregoing method of manufacture is set forth and claimed in my application Serial No. 507,344, filed November 12, 1965, as a continuation-in-part of my application Serial No. 392,991, now abandoned, filed August 7, 1964, a division of my aforementioned application Serial No. 140,815, now abandoned, filed September 26, 1961, and assigned to the assignee of the present invention.

In accordance with the present invention there has been provided a thermionic converter and a method for producing such a converter having greatly improved current density transfer characteristics and therefore an increased power output. The increased power output is on the order of ten times as great as heretofore obtained in converters operating at similar temperatures and at similar vapor pressures. It is observed that with careful construction and with operation at selected temperatures, a power output increase of more than ten times more than that heretofore encountered in thermionic converters can be provided. The resulting increase in efficiency over prior converters is on the order of from percent to 25 percent.

As will be appreciated by those skilled in the art, certain departures may be made from the described embodiments. For example, the preferred materials listed for the device electrodes as well as the system for cooling the anode electrode may be suitably replaced by other known arrangements and materials without departing from the present invention. The present invention primarily relates to a thermionic device wherein an electrode surface or surfaces are largely single crystalline and preferably has an emitting face whose crystal lattice is close packed atomically, whereby the emitter coating adheres well at the converter operating temperatures and exhibits an improved and much higher current electron emission and electron transmission characteristic.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects; and I intend, therefore, in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An energy converter comprising a collector electrode, a metallic emitter electrode with exposed face portions in spaced opposed relation to said collector electrode which have predominantly a crystallographic orientation which is atomically the most closely packed of any crystallographic orientation of the crystal lattice of the metallic emitter, said collector electrode located in spaced relation and opposite said face portions to receive emission therefrom, means providing a temperature differential between said electrodes wherein said emitter temperature is at least 1000 C. to cause thermionic emission of electrons from said emitter electrode to said collector electrode, and an alkali metal atmosphere disposed between said electrodes in at least a partially ionized state acting to neutralize the electron space charge between said electrodes, said alkali metal adhering to said face portions of said emitter electrode to lower the work function of said emitter electrode and cause increased emission density for a given temperature differential and pressure of said gaseous atmosphere, said face portions being characterized by a high work function and low emission compared with other crystallographic orientations in the absence of said gaseous atmosphere, which work function is also high relative to the ionization potential of said alkali metal atmosphere.

2. An energy converter for converting thermal energy to electric energy comprising a collector electrode, an emitter electrode having a surface comprising a single metallic crystal exposing a face in a given crystallographic direction towards the collector electrode, said crystallographic direction being 110 for an emitter electrode having a body centered cubic' lattice, 111 for an emitter electrode having a face centered cubic lattice and being 0001 for an emitter electrode having a hexagonal crystal latice, means providing the temperature differential between said electrodes with the said emitter electrode at a temperature between 1000 and 2500 K., and an alkali metal vapor atmosphere between said electrodes having an ionization potential lower than the normal work function of said emitter electrode, submolecular particles of said alkali metal adhering to said crystal face to provide increased current density from said converter for said given electrode temperature in the presence of said vapor atmosphere.

3. An energy converter for converting thermal energy to electric energy comprising a metallic emitter electrode having an emitting face, a collector electrode located in closely spaced relation with said emitter electrode and oriented opposite said emitter face, means providing temperature differential between said emitter and said collector electrodes wherein said emitter temperature is at least 1600 C. to cause thermionic emission of electrons from the face of said emitter to said collector electrode, and an alkali metal gaseous atmosphere disposed between said electrodes which becomes ionized therebetween to neutralize electronic space charge, the said emitter face exposing predominantly a single crystallographic direction towards said collector characterized by having a high work function in the absence of said alkali metal vapor and as compared with the ionization potential of said vapor, said crystallographic direction being for an emitter electrode having a body centered cubic lattice, 111 for an emitter electrode having a face centered cubic lattice and being 0001 for an emitter electrode having hexagonal crystal lattice, said emitter face causing increased adherence of alkali metal material to said face from said atmosphere for a given temperature of said emitter and alkali metal vapor pressure.

4. An enegry converter for converting thermal energy to electric energy comprising metallic emitter electrodes having a surface comprising a single metallic crystal exposing a face in a given crystallographic direction, said crystallographic direction being 110 for an emitter electrode having a body centered at the lattice, 111 for an emitter electrode having a face centered crystal lattice and being 0001 for an emitter electrode having hexagonal crystal lattice, a collector in close spaced relation opposite said face of said emitter electrode, means for applying thermal energy to said emitter electrode for raising the temperature thereof to at least 1000 C., means for maintaining said collector electrode at a temperature lower than said emitter electrode, and a source of alkali metal vapor communicating with the space between said electrodes for providing ions to neutralize the space charge between said electrodes and having an ionization potential less than the work function of said emitter electrode in the absence of said vapor, said gas ions adhering to both said electrodes and functioning to lower the work function of both said electrodes but lowering the work function of said collector electrode more than said emitter electrode because of the collectors lower temperature, said ions adhering to the said crystal face of the emitter electrode providing a dipole layer with the said crystal face to cause a high density of electron emission from said emitter electrode for given temperatures and vapor pressures.

5. An energy converter for converting thermal energy to electric energy comprising a collector electrode, a

metallic emitter electrode having a surface spaced from and facing said collector electrode and exposing predominantly a single crystallographic direction towards the collector electrode, said crystallographic direction being for an emitter electrode having a body centered cubic lattice, 111 for an emitter electrode having a face centered cubic lattice and being 0001 for an emitter electrode having a hexagonal crystal lattice, means providing a temperature differential between said electrodes with said emitter electrode at a temperature between 1000 and 2500 K., and an alkali metal vapor atmosphere between said electrodes having an ionization potential lower than the normal work function of said emitter electrode, submolecular particles of said alkali metal adhering to said crystal face to provide increased current density from said converter for said given electrode temperature in the presence of said vapor atmosphere.

References Cited by the Examiner UNITED STATES PATENTS 2,162,279 5/ 1939 Herchenrider 29-121 2,179,090 11/1939 Holman 313--329 2,324,505 7/ 1943 Iams 313329 3,123,726 3/1964 Maynard 3104 3,138,725 6/ 1964 Houston 310-4 3,144,569 8/1964 Coles 3104 FOREIGN PATENTS 854,036 11/ 1960 Great Britain.

MILTON O. HIRSHFIELD, Primary Examiner. ORIS L. RADER, J. W. GIBBS, Assistant Examiners. 

1. AN ENERGY CONVERTER COMPRISING A COLLECTOR ELECTRODE, A METALLIC EMITTER ELECTRODE WITH EXPOSED FACE PORTIONS IN SPACED OPPOSED RELATION TO SAID COLLECTOR ELECTRODE WHICH HAVE PREDOMINANTLY A CRYSTALLOGRAPHIC ORIENTATION WHICH IS ATOMICALLY THE MOST CLOSELY PACKED OF ANY CRYSTALLOGRAPHIC ORIENTATION OF THE CRYSTAL LATTICE OF THE METALLIC EMITTER, SAID COLLECTOR ELECTRODE LOCATED IN SPACED RELATION AND OPPOSITE SAID FACE PORTIONS TO RECEIVE EMISSION THEREFROM, MEANS PROVIDING A TEMPERATURE DIFFERENTIAL BETWEEN SAID ELECTRODES WHEREIN SAID EMITTER TEMPERATURE IS AT LEAST 1000*C. TO CAUSE THERMIONIC EMISSION OF ELECTRONS FROM SAID EMITTER ELECTRODE TO SAID COLLECTOR ELECTRODE, AND AN ALKALI METAL ATMOSPHERE DISPOSED BETWEEN SAID ELECTRODES IN AT LEAST A PARTIALLY IONIZED STATE ACTING TO NEUTRALIZE THE ELECTRON SPACE CHARGE BETWEEN SAID ELECTRODES, SAID ALKALI METAL ADHERING TO SAID FACE PORTIONS OF SAID EMITTER ELECTRODE TO LOWER THE WORK FUNCTION OF SAID EMITTER ELECTRODE AND CAUSE INCREASED EMISSION DENSITY FOR A GIVEN TEMPERATURE DIFFERENTIAL AND PRESSURE OF SAID GASEOUS ATMOSPHERE, SAID FACE PORTIONS BEING CHARACTERIZED BY A HIGH WORK FUNCTION AND LOW EMISSION COMPARED WITH OTHER CRYSTALLOGRAPHIC ORIENTATIONS IN THE ABSENCE OF SAID GASEOUS ATMOSPHERE, WHICH WORK FUNCTION IS ALSO HIGH RELATIVE TO THE IONIZATION POTENTIAL OF SAID ALKALI METAL ATMOSPHERE. 